Reverse circulation is a technique which offers may advantages over "normal" circulation when applied to rock drills comprising a surface rotary drill with double wall drill stem and a conventional tri-cone bit, and particularly when applied to work drills comprising a percussive tool suspended down the hole, herein referred to as down-the-hole (DTH) hammer drills. The main advantage is speed of penetration and removal of cuttings. In a reverse circulation system, rock cuttings and debris are forced up through a hollow passageway in the drill itself and the drill string to the surface from the bottom face of the hole by the action of recirculated drilling fluid under pressure. The drilling fluid, which may for example be compressed air, or air/water, or mud in the case of a rotary drill, is circulated down the annular space between the wall of the hole and the drill string, or alternatively down an annular space inside the drill string. It is more preferred to circulate fluid down an annular space in the drill string as this preserves the integrity of the hole wall. Another advantage of the reverse circulation technique is that rock formations are continuously sampled as drilling proceeds, and a representative sample can be collected and monitored at the surface as sample is returned quickly from the bottom of the hole.
Firstly, there are conventional reverse circulation rotary drills which use a tri-cone bit. These can return cuttings directly from the drill face resulting in a geological sample of reasonable purity (e.g. approximately 7-12% contamination). Sample contamination can occur by caving formations or by particles eroded from upper portions of the wall of the hole. Sample is returned after encountering a void so that there is little loss of sample in fractured formations. It is possible to drill about 700-1000 ft below the water table using a standard compressor. However, rotary drills suffer from slow rates of penetration when drilling through hard rock, the cost of the tri-cone bit is high and the bit life is short (e.g. typically about 200-1000 ft drilled).
Secondly, there are the known reverse circulation DTH hammer drills which include a crossover sub-connector spaced from the bit. Although operating costs are lower than in rotary drilling and penetration rates are higher in hard rock, the geological sample may be of questionable purity with contamination levels of about 10-20%. Significant contamination occurs during passage of the debris and cuttings between the face of the bit and the crossover sub-connector externally of the bit, before the debris enters the central bore where it is carried to the surface. Also, the return flow of air is affected by ground water in the hole and the drill tends to flood out at about 400-500 ft below the water table, unless a booster compressor is used. When a void is encountered, there is an interruption of the return of sample until such time as the crossover sub-connector has penetrated beyond the void and resealed in the hole--generally this results in the loss of about 6-8 ft of sample. In general, the known drills are prone to blockage in soft formations and overburden.
It has been proposed in U.S. Pat. No. 3,795,283 to exhaust fluid through peripheral bores of the bit and to return fluid and debris via a central throughbore in the bit and drill in a reverse circulation DTH drill also driven by a conventional surface rotary drilling table. However, this arrangement suffers from a number of disadvantages:
It is still basically a rotary drill. PA1 The drill comprises a comparatively small piston which reciprocates in an annular chamber to repeatedly strike a longer, heavier, anvil to which a bit is secured. Thus, the piston does not directly strike the bit. PA1 The bit employed is a conventional drifter chisel bit adapted for DTH drilling. Thus it is possible to some extent for fluid to exhaust at the bit face to the external periphery of the bit, as there is no effective seal with the sides of the hole, resulting in some loss of sample and loss of "lift" in the central throughbore. PA1 A significant portion of the exhaust fluid is permanently diverted to the central throughbore in the bit to provide "lift" by means of a jet action. Blockage of the bit face exhaust outlets is thus more likely to occur. PA1 At the bit face, the return or throughbore outlet is axially aligned as opposed to being off center. The outlet is thus more likely to block with large size cuttings and pieces of core which are not broken up. PA1 The structure of the bit is inherently weak, there being sixteen bit face exhaust outlets in the peripheral region of the bit. PA1 a stem portion slidably mountable within the chuck, and adapted to extend into the drill, PA1 a body portion extending from the stem portion to terminate in a bit face adapted to support abrading means, PA1 main fluid exhaust means defined by a duct associated with the stem portion and passing through the body portion to a plurality of exhaust outlets defined in the bit face, PA1 a return outlet defined by the body portion and opening to the bit face, which is in fluid communication with the central return passageway in the drill, and PA1 restricting means defined by said body portion adapted to reduce the flow of exhausted fluid from the exhaust outlets to an annulus defined in use between a hole bore and the drill, whereby substantially all exhausted fluid is circulated from the exhaust outlets to the central return passageway via the return outlet in front of the face of the bit. PA1 a stem portion slidably mountable within the chuck, and adapted to extend into the drill, PA1 a body portion extending from the stem portion to terminate in a bit face adapted to support abrading means, PA1 main fluid exhaust means defined by a duct associated with the stem portion and passing through the body portion to a plurality of exhaust outlets defined in the bit face, wherein at least one of the exhaust outlets is directed inwardly towards the center of the face of the bit, and PA1 a return outlet defined by the body portion and opening to the bit face which is in fluid communication through the bit with the central return passageway in the drill. PA1 an outer wear sleeve; PA1 a backhead assembly located at one end of the outer wear sleeve for connecting the drill apparatus to a double-walled drill string and to a source of pressure fluid to actuate the drill; PA1 a fluid diverter mounted inside the outer wear sleeve adjacent to the backhead; PA1 an inner tube concentric with the outer wear sleeve and extending into the fluid diverter defining at least part of a central return passageway in the drill apparatus; PA1 a bit located by a chuck mounting at the other end of the outer wear sleeve slidably mounted on the said inner tube in an annular chamber defined by the outer wear sleeve, by the inner tube, the diverter at one end, and the bit at the other end wherein the bit defines main fluid exhaust means communicating directly between the chamber and the face of the bit; PA1 an inner sleeve mounted inside the outer wear sleeve towards said one end of the chamber adjacent to the diverter defining a fluid communication passage between the diverter and the chamber via porting means; PA1 a piston slidably disposed in the said chamber with respect to the inner sleeve to cooperate with the porting means and mounted on the inner tube to reciprocate within the chamber so as to repeatedly deliver a blow to the bit. PA1 1. It is capable of high penetration rates, even in hard rock. PA1 2. Because drilling fluid is exhausted almost entirely to the face of the-bit and passes up through the central return passageway, this allows for a virtually contamination-free sample, since debris and cuttings are returned to the surface entirely inside the apparatus. PA1 3. Because the exhaust and return openings are located in the face of the bit, drilling in a ground water-filled hole can proceed to much greater depths before the water pressure adversely affects the drill's performance. In fact, extra water pressure can actually assist the return of sample as there is no opposition between the return flow of sample and of water to the surface. PA1 4. When exhaust openings communicate directly with the return openings at the bit face, and/or when the exhaust openings are inwardly-directed towards the return openings, the problem of mud-plugging when drilling in overburden or soft formations such as mud or clay is largely overcome. PA1 5. The drill and bit may be adapted to cooperate to open return by-pass passageways when the bit drops forward on entering a void or cavity in the formation being drilled. The additional by-pass fluid flow serves to keep the debris and cuttings in suspension in the return tube, thus preventing fall back blockage and ensuring no loss of sample. PA1 6. Because fluid and debris are not exhausted above the periphery of the drill bit, the periphery may be a substantially continuous surface thus effectively sealing the bottom hole cavity, or cutting down the ingress of ground water in the hole to the cavity. The tendency to flood out the drill is greatly reduced, and water gathering at the bottom of the hole can be flushed out more quickly than with conventional drills.
It is an object of present invention to provide a reverse circulation DTH hammer drill with improved efficiency and performance in both overburden and hard rock, as compared to prior art drills.